Kiwifruit-like Persistent Luminescent Nanoparticles with High

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Kiwifruit-Like Persistent Luminescent Nanoparticles with High-Performance and in Situ Activable Near-Infrared Persistent Luminescence for Long-Term in Vivo Bioimaging Xia-Hui Lin, Liang Song, Shan Chen, Xiao-Feng Chen, JingJing Wei, Jingying Li, Guoming Huang, and Huang-Hao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13920 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Kiwifruit-Like Persistent Luminescent Nanoparticles with High-Performance and in Situ Activable NearInfrared Persistent Luminescence for Long-Term in Vivo Bioimaging Xia-Hui Lin,† Liang Song,† Shan Chen,† Xiao-Feng Chen,† Jing-Jing Wei,‡ Jingying Li,‡ Guoming Huang,*‡ and Huang-Hao Yang*† †

MOE Key Laboratory for Analytical Science of Food Safety and Biology, State Key Laboratory

of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China. ‡

College of Biological Science and Engineering, Fuzhou University, Fuzhou 350116, P. R.

China. KEYWORDS: persistent luminescence nanoparticles, X-ray activation, chromium-doped zinc gallate, high temperature calcination, bioimaging

ABSTRACT: The persistent luminescence nanoparticles (PLNPs) have great potential for bioimaging because they can eliminate the tissue autofluorescence and improve the signal-to-

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noise ratio (SNR) significantly. High temperature calcination is a necessary process for the PLNPs to achieve high luminescence intensity and long afterglow time. However, high temperature calcination usually results in uncontrollable morphology and poor homogeneity of PLNPs, which greatly limit their applications. Therefore, there is still a high demand to find a suitable method for synthesizing PLNPs with high luminescence intensity and long afterglow time but maintaining their monodispersed morphology. Herein, we report a facile silica template method to synthesize PLNPs with a kiwifruit-like structure that can tolerate high temperature calcination. The as-prepared kiwifruit-like SiO2@ZnGa2O4:Cr3+@SiO2 PLNPs have enhanced near-infrared persistent luminescence, uniform morphology and size, and good biocompatibility. Moreover, the SiO2@ZnGa2O4:Cr3+@SiO2 PLNPs can be repeatedly activated by soft X-ray in situ, and emit near-infrared persistent luminescence with long decay time, holding great potential for deep tissue and long-term in vivo bioimaging. We believe this study will open new perspectives for synthesizing high performance PLNPs for optical imaging and diversified applications.

1. Introduction Persistent luminescent nanoparticles (PLNPs), a newly developed generation of optical probes, have aroused intensive attention in optical diagnosis and luminescence bioimaging.1-5 They allow optical activation before bioimaging and remain luminescent without the need for external illumination, thereby eliminating the autofluorescence from biological tissues and improving the signal-to-noise ratio (SNR) significantly.3 For example, Cr3+-doped zinc gallate (ZnGa2O4:Cr3+) has emerged as a promising optical nanoprobe for bioimaging applications due to its nearinfrared (NIR) persistent luminescence that emitted within the tissue-transparency window.6-9 Normally, the PLNPs are activated by ultraviolet or orange/red light-emitting diodes (LEDs)


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light sources, however, poor penetration depth of these light sources usually limited the applications of PLNPs in body.7 Recently, X-rays were demonstrated that can use to activate the luminescent centers of various nanophosphors.10-15 The scattering of X-rays in tissues is negligible, and X-ray photons have much larger penetration depth in body tissues compared to ultraviolet or visible light. Our group have recently reported that soft X-ray photons were able to serve as a new form of activation source to activate SrAl2O4:Eu2+ PLNPs in deep tissue in animals.13 The combination of negligible scattering of X-rays in tissues, and the high tissue penetration of NIR photons emitted from appropriate nanoprobes, might offer the opportunity to achieve deep tissue optical imaging with unprecedented spatial resolution.16 Despite the great promise of PLNPs, some challenges remain to be addressed. In general, the PLNPs should possess high luminescence performance, controllable size, regular morphology, and easy modification for practical biomedical applications.3, 17-19 High temperature calcination is a necessary process for obtaining PLNPs with good luminescence properties.3 Increase of calcination temperature can lead to an increase of the persistent luminescence intensity and decay time.20-23 However, the high temperature annealing generally produces PLNPs with large size and poor dispersibility.23 Furthermore, the calcination of PLNPs at high temperature can result in the removal of surfactants and functional groups, making PLNPs tend to aggregate, which hamper the further functionalization of PLNPs. The tradeoff between the luminescence performance and morphology is not well resolved. Therefore, the development of new design approaches to achieve PLNPs with uniform morphology and good luminescence performance is still highly desired. In this work, we reported a novel silica template-assisted method to synthesize monodisperse PLNPs with high persistent luminescence performance, controllable size, and defined


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morphology. We used SiO2 nanoparticles as the template to in situ synthesize ZnGa2O4:Cr3+ and then further coated a SiO2 shell to fabricate kiwifruit-like SiO2@ZnGa2O4:Cr3+@SiO2 (SZGOS) nanoparticles (Scheme 1). The size of the SZGOS nanoparticles was able to be easily tuned by controlling the size of SiO2 core and the thickness of SiO2 shell. The kiwifruit-like structure made the internal ZnGa2O4:Cr3+ tolerate higher temperature calcination, therefore endowed the ZnGa2O4:Cr3+ with better persistent luminescence performance. Moreover, this kiwifruit-like structure can generate steric constraints against the grain growth, which prevent the agglomeration of the whole nanoparticle during the high temperature calcination. Therefore, the SZGOS nanoparticles had enhanced persistent luminescence while also maintained the monodisperse morphology after the calcination. Furthermore, we demonstrated that the highly dispersed SZGOS nanoparticles were able to be repeatedly activated by not only ultraviolet light or red LEDs, but also soft X-ray, showing great potential in deep tissue and long-term in vivo bioimaging.

Scheme 1. Schematic illustration of the synthesis of SZGOS nanoparticles and their applications for long-term in vivo bioimaging.


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2. Results and Discussion 2.1. Synthesis and characterization of the SZGOS nanoparticles We first prepared ~100 nm SiO2 nanospheres through a modified Stöber method.24 The transmission electron microscopy (TEM) image revealed that the as-synthesized SiO2 were nearly spherical with narrow size distribution (Figure 1a).

We then synthesized

SiO2@ZnGa2O4:Cr3+ nanoparticles using SiO2 nanospheres as hard templates.25-26 Zn2+, Ga3+ and Cr3+ metal ions were first mixed with ~100 nm SiO2 nanospheres. The zeta potential of SiO2 nanospheres was –28.2 ± 3.5 mV, therefore the metal ions easily attached to SiO2 nanospheres through the electrostatic adsorption. During this mixing process, the solution rapidly became turbid white when the pH was adjusted to ~7.5, which ascribed to the formation of ZnGa2O4:Cr3+ precursors on the surface of silica nanospheres under alkaline conditions. The mixture then was treated with a hydrothermal process. The TEM image confirmed the core-shell structure of the as-resulted SiO2@ZnGa2O4:Cr3+ (SZGO) nanoparticles, and the size was shown as about 120 nm (Figure 1b). The dynamic light scattering (DLS) measurements showed that the hydrodynamic diameter of SZGO nanoparticles was about 134 nm, exhibiting an increase compared to the 110 nm of SiO2 templates (Supporting Information, Figure S1). We further coated the SZGO nanoparticles with a silica layer through wet chemical coating. The TEM image revealed that the thickness of silica shell was about 17 nm, and the size of the whole SZGOS nanoparticles was about 158 nm (Figure 1c). DLS analyses showed that the hydrodynamic diameters of SZGOS nanoparticles increased to 180 nm after the surface coating with the silica shell (Figure S1). It should be noted that the thickness of the silica layer can be tuned by the added concentration of TEOS and ammonia water, and therefore the size of the SZGOS nanoparticles can be further regulated by the thickness of silica layer (Figure S2). The X-ray powder diffraction (XRD)


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pattern of the obtained SZGOS nanoparticles revealed a pure spinel phase structure of ZnGa2O4:Cr3+ (Figure 1d). In addition, a clear lattice space of 0.251 nm corresponding to the (311) plane of ZnGa2O4:Cr3+ was observed in the high-resolution TEM image of SZGOS nanoparticles (Figure 1e). These results confirmed the successful synthesis of ZnGa2O4:Cr3+ nanoparticles between the silica interlayers. The energy-dispersive X-ray (EDX) element mapping analysis further confirmed the kiwifruit-like structure of the SZGOS nanoparticles that ZnGa2O4:Cr3+ nanoparticles located between the silica interlayers (Figure 1f).

Figure 1. TEM images of (a) SiO2 nanospheres, (b) SZGO nanoparticles, and (c) SZGOS nanoparticles. (d) XRD pattern, (e) high-resolution TEM image, (f) EDX element mapping analysis of the SZGOS nanoparticles.


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2.2. High temperature calcination of the SZGOS nanoparticles The high temperature sintering is a necessary process to obtain the PLNPs with designed compositions and good luminescence performance.1, 3 Higher calcination temperature can lead to better persistent luminescence performance of the PLNPs. We synthesized bare ZnGa2O4:Cr3+ nanoparticles as a control sample. The as-synthesized ZnGa2O4:Cr3+ nanoparticles had particle sizes about 20 nm as revealed by TEM (Figure 2a). The XRD pattern confirmed the phase structure of the ZnGa2O4:Cr3+ nanoparticles (Figure S3). However, under high temperature calcination, the temperature significantly influenced the size and morphology of the ZnGa2O4:Cr3+ nanoparticles. The bare ZnGa2O4:Cr3+ nanoparticles aggregated obviously and grew to larger size particles up to several microns with uncontrollable morphology when the temperature was increased from 600 °C to 950 °C (Figure 2b-d). The biomedical applications require PLNPs with small size and good monodispersity. The aggregated products cannot meet the requirement of in vivo biomedical applications. Then we investigate the influence of the high temperature on the size and morphology of SZGOS nanoparticles. Remarkably, the SZGOS nanoparticles maintained their uniform size and morphology even at the sintering temperature up to 950 °C (Figure 2e-h). These results suggested that the introduction of the silica templates can effectively protect the intercalated ZnGa2O4:Cr3+ nanoparticles from agglomeration during the high temperature sintering by generating steric constraints against the grain growth. In previous work, mesoporous silica nanoparticles were showed to be effective templates to synthesize PLNPs with good morphology and stability,27 which also confirmed that the protect effects of silica shells against agglomeration.


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Figure 2. The TEM images of (a) ZnGa2O4:Cr3+ nanoparticles without calcination, (b, c, d) ZnGa2O4:Cr3+ nanoparticles after calcination for 4 h in air at 600 oC, 800 oC, and 950 oC, respectively. (e) SZGOS nanoparticles without calcination, (f, g, h) SZGOS nanoparticles after calcination for 4 h in air at 600 oC, 800 oC, and 950 oC, respectively.

Afterwards, we investigated the persistent luminescence performance of SZGOS nanoparticles under the different sintering temperature. Notably, both the luminescence intensity and afterglow time of SZGOS nanoparticles were significantly improved with the increasing of sintering temperature (Figure 3). The luminescence intensity and afterglow time of SZGOS nanoparticles after calcination at 950 oC showed increases of 1050% and 1475%, respectively, compared to that at 600 oC. The intrinsic lattice defects of PLNPs play the key role in the generation of persistent luminescence.3 Higher calcination temperature can generally increase the population of intrinsic lattice defects,3 therefore can improve the persistent luminescence properties of PLNPs. The XRD patterns confirmed that the SZGOS nanoparticles still possess ZnGa2O4:Cr3+ phase without impurity after high temperature calcination (Figure S4). These results indicated that the SZGOS nanoparticles we designed could effectively solve the tradeoff between


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morphology and performance of PLNPs. Since the SZGOS nanoparticles after calcination at 950 o

C exhibited the best performance, we only investigated this sample in the subsequent

experiments. To the best of our knowledge, it is the first demonstration that PLNPs possess a regular morphology and strong luminescence properties after 950 oC calcination.

Figure 3. (a) Persistent luminescence and (b) decay curves of the SZGOS nanoparticles (15 mg) activated by soft X-ray after calcination at different temperatures.

2.3. Activation of SZGOS nanoparticles by soft X-ray The SZGOS nanoparticles could be activated by ultraviolet or red LEDs, and the resulting persistent luminescence signals were observed for more than 24 h (Figure S5). However, for in vivo biomedical applications, the ultraviolet or visible light sources are usually limited due to their poor penetration in tissue. Compared to ultraviolet or visible light, soft X-ray light source possesses the advantage of deep tissue penetration. As demonstrated above, after being illuminated with the soft X-ray, the SZGOS nanoparticles generated an emission band in the range of 600-750 nm with the peak at 696 nm corresponding to the spin-forbidden 2E →4A2


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transition,5, 28 confirming that the SZGOS nanoparticles were able to be activated by soft X-ray (Figure 3a). Moreover, the afterglow time of SZGOS nanoparticles was more than 24 h. To further evaluate the capability of the SZGOS nanoparticles for X-ray activated NIR luminescence imaging, we collected the NIR luminescence images at different time points using a CCD camera. After 60 s illumination with the soft X-ray, the SZGOS nanoparticles showed persistent luminescence imaging signals for more than 24 h, and these signals could be repeatedly activated (Figure 4a). The decay curves showed that the SZGOS nanoparticles could quickly absorb X-ray energy and reach saturation, then slowly released afterglow in the absence of illumination (Figure 4b). More importantly, over five X-ray activation cycles, the decay curves of SZGOS nanoparticles remained stable without significant changes, indicating that the SZGOS nanoparticles had outstanding photostability under repeated soft X-ray illumination.

Figure 4. (a) NIR luminescence images of the SZGOS nanoparticles (15 mg) obtained by a CCD camera at different time points after 60 s irradiation with a soft X-ray source (40 kV tube voltage, 70 µA tube current). (b) Decay curves over five cycles of the SZGOS nanoparticles using soft X-ray as the activation light source.


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The in vivo PLNPs imaging was usually limited by the emission lifetime of the PLNPs after an ex-vivo activation due to limited penetration depth of ultraviolet or visible activation sources. The SZGOS nanoparticles can be repeatedly activated by soft X-ray with deep tissue penetration, which means that the SZGOS nanoparticles can be repeatedly activated in situ in deep tissue, therefore the luminescence imaging might be no longer limited by the afterglow lifetime of the PLNPs. To further confirm the potential use of SZGOS nanoparticles for deep-tissue imaging, we conducted a simulation experiment by placing a piece of pork (~1 cm thickness) between the SZGOS nanoparticles and light source (Figure 5a, b). The persistent luminescence was hardly detected when 430 nm light was used as the activation source (Figure 5c), demonstrating that the 430 nm visible light was difficult to penetrate the pork slab to activate the SZGOS nanoparticles. In contrast, the persistent luminescence signals were clearly observed, and lasted for more than 30 min after the irradiation of soft X-ray (Figure 5d). Additionally, these signals could be repeatedly activated showing no obvious attenuation. These results proved that soft X-ray with strong penetrating abilities was able to use as a highly effective activation light source for deeptissue luminescence imaging.


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Figure 5. (a) Photograph and (b) X-ray imaging of the experimental setup that a ~1 cm thick pork covering on the SZGOS nanoparticles. (c) NIR persistent luminescence image of the SZGOS nanoparticles after treated with 430 nm light activation. (d) NIR persistent luminescence images of the SZGOS nanoparticles after the activation of soft X-ray over three times.

2.4. Functionalization of SZGOS nanoparticles High temperature sintering can lead to the loss of surface groups of the PLNPs, resulting in agglomeration and poor dispersibility of the PLNPs. Due to the protection of the silica templates, the SZGOS nanoparticles still maintained their size and morphology after the sintering. Furthermore, the introduction of the silica shell would be beneficial for the surface modification. We then functionalized the SZGOS nanoparticles with polyethylene glycol (PEG)


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to render these nanoparticles water-soluble for biomedical applications.7 Surface modification of the SZGOS with PEG was confirmed by the Fourier transform infrared (FTIR) spectrum. In the FTIR spectra, the typical peaks of C−O−C (1094 cm−1), amide bond (1656 cm−1), and C−H (2847 cm−1 and 2921 cm−1) bond vibration were observed, indicating the successful PEGylation of SZGOS nanoparticles (Figure 6a). The obtained PEGylated SZGOS nanoparticles could be stably dispersed in various solutions including water, phosphate buffer saline (PBS), cell culture medium (RMPI), fetal bovine serum (FBS) without aggregation over at least two weeks (Figure 6b). A key requirement for bioimaging applications is that the nanomaterials must be biocompatible.29-31 To evaluate the cytotoxicity of SZGOS nanoparticles, we performed the CCK-8 assay. After being incubated with HepG2 cells for 24 h, the SZGOS nanoparticles showed no significant influence on cell viability even at the highest concentration, suggesting the good biocompatibility of SZGOS nanoparticles (Figure 6c).

Figure 6. (a) FTIR spectrum of the PEGylated SZGOS nanoparticles. (b) Photographs of the PEGylated SZGOS nanoparticles dispersed in various solutions. (c) Cell viability of HepG2 cells after incubated with PEGylated SZGOS nanoparticles with different concentrations for 24 h.


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2.5. In vivo NIR luminescence imaging of SZGOS nanoparticles Normally, NIR persistent luminescence requires ultraviolet light to provide the energy. However, owing to the rather limited tissue penetration depth of ultraviolet or visible light, these PLNPs had to be precharged in vitro. Furthermore, living tissues typically exhibit strong autofluorescence under ultraviolet or visible light irradiation (Figure S6), which might also hinder the applications of these light sources in optical bioimaging. Although there have been developed NIR activation of PLNPs,32 but its penetration is still limited and the part of energy easily absorbed by tissue and water, causing the less energy delivered to PLNPs. To address these problem, we used soft X-ray as irradiation source. The above experiments have demonstrated that the SZGOS nanoparticles could be repeatedly activated in situ. We further performed in vivo NIR luminescence imaging using BALB/c nude mice as models. We first intravenously injected the mice with SZGOS nanoparticles (dose of 200 µL, 1 mg mL-1). After 12 h, the mice were activated by soft X-ray for 60 s. Since the nanomaterials were highly taken up by the hepatic Kupffer cells, leading to their high accumulation in the liver,33-35 we focused on the liver as the targeting region for assessing the in vivo luminescence imaging effects of SZGOS nanoparticles. The persistent luminescence images showed that the major signals of persistent luminescence were detected in the liver region for more than 2 h, suggesting that the SZGOS nanoparticles could eliminate the tissue autofluorescence (Figure 7). When the mice were irradiated again by soft X-ray, the persistent luminescence signals reactivated sufficiently, confirming that the SZGOS nanoparticles could be repeatedly activated in situ for long-term in vivo imaging. It is worth noting the dose of X-ray we used was only 0.18 Gy, which is much lower than the fraction doses used in clinical radiotherapy.36-37


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Figure 7. Persistent luminescence images of mice after intravenous injection of SZGOS nanoparticles (200 µL, 1 mg mL-1) and soft X-ray activation (40 kV, 70 µA, 60s).

3. Conclusion In conclusion, we have successfully synthesized the kiwifruit-like SZGOS nanoparticles using a silica template-assisted method. The SZGOS nanoparticles showed excellent thermal stability that maintained their uniform morphology without agglomeration under high temperature sintering. Moreover, the SZGOS nanoparticles possessed outstanding persistent luminescence performance after the calcination at 950 oC. They emitted NIR persistent luminescence in the tissue-transparency window, and were able to be repeatedly activated by soft X-ray in situ, providing highly sensitive optical detection without autofluorescence signal from living tissues.


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We believe that this study not only developed an optical probe for long-term in vivo bioimaging, but also provided a novel synthesis strategy for fabricating PLNPs with high optical performance, controllable size, defined morphology, and good biocompatibility.

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Experimental details, supporting figures (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. U1505221, 21635002, and 81501461), Natural Science Foundation of Fujian Province of China (No. 2015H6011), the Program for Changjiang Scholars and Innovative Research Team in University


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Table Of Contents (TOC)


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Kiwifruit-like Persistent Luminescent Nanoparticles with High

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